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Ultrafast dynamics of femtosecond laser-induced high spatial frequency periodic structures on silicon surfaces
Femtosecond laser-induced periodic surface structures (LIPSS) have been extensively studied over the past few decades. In particular, the period and groove width of high-spatial-frequency LIPSS (HSFL) is much smaller than the diffraction limit, making it a useful method for efficient nanomanufacturing. However, compared with the low-spatial-frequency LIPSS (LSFL), the structure size of the HSFL is smaller, and it is more easily submerged. Therefore, the formation mechanism of HSFL is complex and has always been a research hotspot in this field. In this study, regular LSFL with a period of 760 nm was fabricated in advance on a silicon surface with two-beam interference using an 800 nm, 50 fs femtosecond laser. The ultrafast dynamics of HSFL formation on the silicon surface of prefabricated LSFL under single femtosecond laser pulse irradiation were observed and analyzed for the first time using collinear pump-probe imaging method. In general, the evolution of the surface structure undergoes five sequential stages: the LSFL begins to split, becomes uniform HSFL, degenerates into an irregular LSFL, undergoes secondary splitting into a weakly uniform HSFL, and evolves into an irregular LSFL or is submerged. The results indicate that the local enhancement of the submerged nanocavity, or the nanoplasma, in the prefabricated LSFL ridge led to the splitting of the LSFL, and the thermodynamic effect drove the homogenization of the splitting LSFL, which evolved into HSFL.
HSFL is usually induced by multiple femtosecond laser pulses, which makes the formation mechanism more complex because of several factors, such as deposited debris, a disturbed light field of the subsequent laser, thermal accumulation, and hydrodynamic effects. In this study, we demonstrated for the first time the evolution of HSFL formation induced by a single femtosecond laser pulse using collinear pumped-probe imaging method. A regular LSFL with a period of 760 nm was fabricated in advance on a silicon surface with two-beam interference from an 800 nm femtosecond laser. The ultrafast dynamics of the HSFL formation on the silicon surface of a prefabricated LSFL under single-femtosecond laser pulse irradiation were studied. A common feature is that the evolution of the surface structure undergoes five sequential stages: the LSFL begins to split, becomes a uniform HSFL, degenerates into an irregular LSFL, undergoes secondary splitting into a weakly uniform HSFL, and evolves into an irregular LSFL or is submerged. By analyzing these results, it was proposed that the splitting of LSFL was caused by the local enhancement of the submerged nanocavities and the filled amorphous silicon in the transient nanogrooves in the middle of the prefabricated LSFL ridge, and the thermodynamic effect drove the homogenization of the splitting LSFL and evolved into HSFL.
(From:https://www.oejournal.org/article/)
